A primary explosive is an explosive that is extremely sensitive to stimuli such as impact, friction, heat, static electricity, radio frequency, or electromagnetic radiation. A relatively small amount of energy is required for initiation of a primary explosive. Generally, primary explosives are considered to be those compounds that are more sensitive than Pentaerythritol tetranitrate (PETN). Primary explosives are often used in detonators or to trigger larger charges of less sensitive secondary explosives. For example, in the oil and gas industry, more standard primary explosive detonators are used than any other detonator types. Such detonators are typically used in connection with perforating technology to blast holes into steel pipes downhole.
FIG. 1 shows a conventional primary explosive (1) ohm detonator. The oil industry prefers using resistorized primary explosive detonators requiring resistors in each lead of the 1 Ohm detonator (not shown in FIG. 1). The primary explosive detonator typically uses a one (1) Ohm electric match with an ignition mixture coating (100). Electric current is passed through the match causing Joule heating that in turn causes the ignition mixture (100) to ignite. The ignition mixture (100) causes the Lead Styphnate (102) to detonate, which in turn causes the Lead Azide (104) to detonate, resulting in the final explosive powder Research Department Explosive/High Melting Explosive (RDX/HMX) or Hexanitrostilbene (HNS) (106) to detonate. The ignition mixture (100), Lead Styphante (102), and Lead Azide (104) are classified as primary explosives, while RDX, HMX or HNS (106) are classified as secondary explosives. The sensitivity of each chemical is in decreasing order from electric match, primary explosive, and then to secondary explosive.
Because primary explosive detonators are very sensitive to stray voltage exposure, electrostatic discharge (ESD), and radio frequency (RF), they can often easily be triggered to explode, causing unsafe environments in an oil and gas setting. For example, it would not take much more than 1 volt of stray voltage exposure to trigger detonation of the primary explosive detonator shown in FIG. 1. The typical no-fire for a fuse such as that shown in FIG. 1 is 200 mA, while all-fire is specified as 800 mA. With a 1 ohm electric match, it takes approximately only 0.2V across the match to reach 200 mA no-fire current and only 0.8V across the match to reach 800 mA all-fire.
As an alternative, Exploding Bridge Wire (EBW) and Exploding Foil Initiator (EFI) detonators are highly resistant to ESD, RF, and stray voltage exposure. EBW and EFI are more expensive to manufacture, and because of the cost, these detonators are mostly used in high tier oil industry applications.
Accordingly, what is needed is primary explosive detonator safer under exposure to RF and stray voltage which does not greatly increase the cost to implement.